Ro-32-2241

Ro 32-2241 is a bisindolylmaleimide that selectively inhibits protein kinase C (PKC) as compared with other protein kinases. Experiments were carried out to examine its potential as a multidrug resistance-reversing agent. Ro 32-2241 inhibited efflux, and increased accumulation, of [3H]-daunomycin in multidrug-resistant (MDR) KB-8-5 and KB-8-5-11 cells and had no effect on drug-sensitive KB-3-1 cells. Ro 32-2241 completely reversed the doxorubicin resistance of KB-8-5 and KB-8-5-11 cells, showing no effect on the sensitivity of drug-sensitive KB-3-1 cells. The potency of Ro 32-2241 was comparable with that of cyclosporin A and better than that of verapamil, known modulators of multidrug resistance. Ro 32-2241 also completely reversed the taxol resistance of KB-8-5 cells and partially reversed the resistance of KB-8-5-11 cells. Vinblastine resistance was also partially reversed. Mechanistic experiments were carried out to determine whether Ro 32-2241 interacted with P-glycoprotein (Pgp) directly. Increased efflux of [14C]-Ro 32-2241 was seen with the more resistant KB-8-5-11 cells (although the percentage effluxed was very low as compared with [3H]-daunomycin), suggesting that Ro 32-2241 can act as a substrate for Pgp. Direct interaction of Ro 32-2241 with Pgp was confirmed by demonstration that it inhibited binding of [3H]-azidopine to Pgp in KB-8-5-11 membranes. In conclusion, Ro 32-2241, acting directly on Pgp (rather than, or in addition to, an effect on PKC), is effective in reducing or reversing resistance to doxorubicin, taxol and vinblastine in human tumour cells with a clinically relevant degree of MDR. However, results of in vivo experiments conducted to investigate the effects of Ro 32-2241 on resistance to doxorubicin suggest that it may not be possible to achieve sufficiently high levels of Ro 32-2241 in vivo to modulate MDR.